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Communications Design Conference 2 October 2003

- 1 - Kinney Consulting [email protected]

ZigBee Technology: Wireless Control that Simply

Works

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ZigBee Technology: Wireless Control that SimplyWorks

Why is ZigBee needed? There are a multitude of standards that address mid to high data rates for voice,

PC LANs, video, etc. However, up till now there hasnt been a wireless

network standard that meets the unique needs of sensors and control devices.

Sensors and controls dont need high bandwidth but they do need low latency

and very low energy consumption for long battery lives and for large device

arrays.

There are a multitude of proprietary wireless systems manufactured today tosolve a multitude of problems that also dont require high data rates but do

require low cost and very low current drain.

These proprietary systems were designed because there were no standards that

met their requirements. These legacy systems are creating significant

interoperability problems with each other and with newer technologies.

The ZigBee Alliance is not pushing a technology; rather it is

providing a standardized base set of solutions for sensor andcontrol systems.

The physical layer was designed to accommodate the need for a low cost yet

allowing for high levels of integration. The use of direct sequence allows the

analog circuitry to be very simple and very tolerant towards inexpensive

implementations.

The media access control (MAC) layer was designed to allow multiple

topologies without complexity. The power management operation doesnt

require multiple modes of operation. The MAC allows a reduced functionalitydevice (RFD) that neednt have flash nor large amounts of ROM or RAM. The

MAC was designed to handle large numbers of devices without requiring them

to be parked.

The network layer has been designed to allow the network to spatially grow

without requiring high power transmitters. The network layer also can handle

large amounts of nodes with relatively low latencies.


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ZigBee is poised to become the global control/sensor network

standard. It has been designed to provide the following

features: Low power consumption, simply implemented

Users expect batteries to last many months to years! Consider that a typical

single family house has about 6 smoke/CO detectors. If the batteries for

each one only lasted six months, the home owner would be replacing

batteries every month!

Bluetooth has many different modes and states depending upon your latency

and power requirements such as sniff, park, hold, active, etc.; ZigBee/IEEE

802.15.4 has active (transmit/receive) or sleep. Application software needs

to focus on the application, not on which power mode is optimum for each

aspect of operation.

Even mains powered equipment needs to be conscious of energy. Consider afuture home with 100 wireless control/sensor devices,

Case 1: 802.11 Rx power is 667 mW (always on)@ 100 devices/home &

50,000 homes/city = 3.33 megawatts

Case 2: 802.15.4 Rx power is 30 mW (always on)@ 100 devices/home &

50,000 homes/city = 150 kilowatts

Case 3: 802.15.4 power cycled at .1% (typical duty cycle) = 150 watts.

ZigBee devices will be more ecological than its predecessors saving

megawatts at it full deployment.

Low cost (device, installation, maintenance)

Low cost to the users means low device cost, low installation cost and low

maintenance. ZigBee devices allow batteries to last up to years using

primary cells (low cost) without any chargers (low cost and easy

installation). ZigBees simplicity allows for inherent configuration and

redundancy of network devices provides low maintenance.

High density of nodes per network

ZigBees use of the IEEE 802.15.4 PHY and MAC allows networks to

handle any number of devices. This attribute is critical for massive sensor

arrays and control networks.

Simple protocol, global implementationZigBees protocol code stack is estimated to be about 1/4

thof Bluetooths or

802.11s. Simplicity is essential to cost, interoperability, and maintenance.

The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868

MHz band in Europe, the 915 MHz band in N America, Australia, etc; and

the 2.4 GHz band is now recognized to be a global band accepted in almost

all countries.


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ZigBee/IEEE 802.15.4 - General Characteristics Dual PHY (2.4GHz and 868/915 MHz)

Data rates of 250 kbps (@2.4 GHz), 40 kbps (@ 915 MHz), and 20 kbps

(@868 MHz)

Optimized for low duty-cycle applications (


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The IEEE 802.15.4 PHY and MAC along with ZigBees

Network and Application Support Layer provide: Extremely low cost

Ease of implementation Reliable data transfer

Short range operation

Very low power consumption

Appropriate levels of security

There are two physical device types for the lowest system cost

To allow vendors to supply the lowest possible cost devices the IEEE standard

defines two types of devices: full function devices and reduced functiondevices

Full function device (FFD)

Can function in any topology

Capable of being the Network coordinator

Capable of being a coordinator

Can talk to any other device

Reduced function device (RFD)

Limited to star topology Cannot become a network coordinator

Talks only to a network coordinator

Very simple implementation

An IEEE 802.15.4/ZigBee network requires at least one full function device as a

network coordinator, but endpoint devices may be reduced functionality devices

to reduce system cost.

All devices must have 64 bit IEEE addresses

Short (16 bit) addresses can be allocated to reduce packet size

Addressing modes:

Network + device identifier (star)

Source/destination identifier (peer-peer)


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Star Topology

PAN

Coordinator

Full functiondevice

Communicationsflow

Peer to Peertopology

Cluster TreeTopology

Full Function Device

Reduced FunctionDevice

CommunicationsFlow


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Frame StructureThe frame structures have been designed to keep the complexity to a minimum

while at the same time making them sufficiently robust for transmission on a

noisy channel. Each successive protocol layer adds to the structure with layer-

specific headers and footers.The IEEE 802.15.4 MAC defines four frame structures:

A beacon frame, used by a coordinator to transmit beacons.

A data frame, used for all transfers of data.

An acknowledgment frame, used for confirming successful frame reception.

A MAC command frame, used for handling all MAC peer entity control

transfers.

The data frame is illustrated below:

The Physical Protocol Data Unit is the total information sent over the air. Asshown in the illustration above the Physical layer adds the following overhead:

Preamble Sequence 4 Octets

Start of Frame Delimiter 1 Octet

Frame Length 1 Octet

The MAC adds the following overhead:

Frame Control 2 Octets

Data Sequence Number 1 Octet

Address Information 4 20 Octets

Frame Check Sequence 2 Octets

In summary the total overhead for a single packet is therefore 15 -31 octets (120

bits); depending upon the addressing scheme used (short or 64 bit addresses).

Please note that these numbers do not include any security overhead.


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Super Frame StructureThe LR-WPAN standard allows the optional use of a superframe structure. The

format of the superframe is defined by the coordinator. The superframe is

bounded by network beacons, is sent by the coordinator (See Figure 4) and is

divided into 16 equally sized slots. The beacon frame is transmitted in the first slot

of each superframe. If a coordinator does not wish to use a superframe structure it

may turn off the beacon transmissions. The beacons are used to synchronize the

attached devices, to identify the PAN, and to describe the structure of the

superframes. Any device wishing to communicate during the contention access

period (CAP) between two beacons shall compete with other devices using a

slotted CSMA-CA mechanism. All transactions shall be completed by the time of

the next network beacon.

For low latency applications or applications requiring specific data bandwidth, the

PAN coordinator may dedicate portions of the active superframe to that

application. These portions are called guaranteed time slots (GTSs). Theguaranteed time slots comprise the contention free period (CFP), which always

appears at the end of the active superframe starting at a slot boundary immediately

following the CAP, as shown in Figure 5. The PAN coordinator may allocate up

to seven of these GTSs and a GTS may occupy more than one slot period.

However, a sufficient portion of the CAP shall remain for contention based access

of other networked devices or new devices wishing to join the network. All

contention based transactions shall be complete before the CFP begins. Also each

device transmitting in a GTS shall ensure that its transaction is complete before

the time of the next GTS or the end of the CFP.


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MAC Data Service Diagrams

MAC Primitives

MAC Data Service MCPS-DATA exchange data packets between MAC and PHY MCPS-PURGE purge an MSDU from the transaction queue

MAC Management Service MLME-ASSOCIATE/DISASSOCIATE network association

MLME-SYNC / SYNC-LOSS - device synchronization MLME-SCAN - scan radio channels MLME- COMM-STATUS communication status MLME-GET / -SET retrieve/set MAC PIB parameters MLME-START / BEACON-NOTIFY beacon management MLME-POLL - beaconless synchronization MLME-GTS - GTS management MLME-RESET request for MLME to perform reset MLME-ORPHAN - orphan device management MLME-RX-ENABLE - enabling/disabling of radio system

Beacon network communication

Non-beacon network communication


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- 10 Kinney Consulting [email protected]

Security

When security of MAC layer frames is desired, ZigBee uses MAC layer security

to secure MAC command, beacon, and acknowledgement frames. ZigBee may

secure messages transmitted over a single hop using secured MAC data frames,but for multi-hop messaging ZigBee relies upon upper layers (such as the NWK

layer) for security. The MAC layer uses the Advanced Encryption Standard (AES)

[10] as its core cryptographic algorithm and describes a variety of security suites

that use the AES algorithm. These suites can protect the confidentiality, integrity,

and authenticity of MAC frames. The MAC layer does the security processing, but

the upper layers, which set up the keys and determine the security levels to use,

control this processing. When the MAC layer transmits (receives) a frame with

security enabled, it looks at the destination (source) of the frame, retrieves the key

associated with that destination (source), and then uses this key to process theframe according to the security suite designated for the key being used. Each key

is associated with a single security suite and the MAC frame header has a bit that

specifies whether security for a frame is enabled or disabled.

When transmitting a frame, if integrity is required, the MAC header and payload

data are used in calculations to create a Message Integrity Code (MIC) consisting

of 4, 8, or 16 octets. The MIC is right appended to the MAC payload. If

confidentiality is required, the MAC frame payload is also left appended with

frame and sequence counts (data used to form a nonce). The nonce is used when

encrypting the payload and also ensures freshness to prevent replay attacks. Uponreceipt of a frame, if a MIC is present, it is verified and if the payload is

encrypted, it is decrypted. Sending devices will increase the frame count with

every message sent and receiving devices will keep track of the last received

count from each sending device. If a message with an old count is detected, it is

flagged with a security error. The MAC layer security suites are based on three

modes of operation. Encryption at the MAC layer is done using AES in Counter

(CTR) mode and integrity is done using AES in Cipher Block Chaining (CBC-

MAC) mode [16]. A combination of encryption and integrity is done using a

mixture of CTR and CBC- MAC modes called the CCM mode.

The NWK layer also makes use of the Advanced Encryption Standard (AES).

However, unlike the MAC layer, the security suites are all based on the CCM*

mode of operation. The CCM* mode of operation is a minor modification of the

CCM mode used by the MAC layer. It includes all of the capabilities of CCM and

additionally offers encryption-only and integrity-only capabilities. These extra

capabilities simplify the NWK layer security by eliminating the need for CTR and


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CBC-MAC modes. Also, the use of CCM* in all security suites allows a single

key to be used for different suites. Since a key is not strictly bound to a single

security suite, an application has the flexibility to specify the actual security suite

to apply to each NWK frame, not just whether security is enabled or disabled

When the NWK layer transmits (receives) a frame using a particular security suiteit uses the Security Services Provider (SSP) to process the frame. The SSP looks

at the destination (source) of the frame, retrieves the key associated with that

destination (source), and then applies the security suite to the frame. The SSP

provides the NWK layer with a primitive to apply security to outgoing frames and

a primitive to verify and remove security from incoming frames. The NWK layer

is responsible for the security processing, but the upper layers control the

processing by setting up the keys and determining which CCM* security suite to

use for each frame.

Similar to the MAC layer frame format, a frame sequence count and MIC may beadded to secure a NWK frame.


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ZigBee Network Model

The ZigBee Network

Coordinator

Sets up a network

Transmits network beacons

Manages network nodes

Stores network node information

Routes messages between paired

nodes

Typically operates in the receive

state

The ZigBee Network Node Designed for battery powered or

high energy savings

Searches for available networks

Transfers data from its

application as necessary

Determines whether data ispending

Requests data from the network

coordinator

Can sleep for extended periods

Mesh

Star

ZigBee End DeviceRFD or FFD

ZigBee Router (FFD)

ZigBee Coordinator (FFD)


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ZigBee Stack

ZigBee Stack System Requirements 8-bit C, e.g., 80c51

Full protocol stack


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Network Layer

The responsibilities of the ZigBee NWK layer include:

Starting a network: The ability to successfully establish a new network.

Joining and leaving a network: The ability to gain membership (join) orrelinquish membership (leave) a network.

Configuring a new device: The ability to sufficiently configure the stack for

operation as required.

Addressing: The ability of a ZigBee coordinator to assign addresses to devices

joining the network.

Synchronization within a network: The ability for a device to achieve

synchronization with another device either through tracking beacons or by

polling.

Security: applying security to outgoing frames and removing security to

terminating frames Routing: routing frames to their intended destinations.

Network Routing OverviewPerhaps the most straightforward way to think of the ZigBee routing algorithm is

as a hierarchical routing strategy with table-driven optimizations applied where

possible.

NWK uses an algorithm that allows stack implementers and application

developers to balance unit cost, battery drain, and complexity in producing

ZigBee solutions to meet the specific cost-performance profile of their

application.

Started with the well-studied public-domain algorithm AODV and Motorolas

Cluster-Tree algorithm and folding in ideas from Ember Corporations GRAd.

Network SummaryThe network layer builds upon the IEEE 802.15.4 MACs features to allow

extensibility of coverage. Additional clusters can be added; networks can be

consolidated or split up.


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Application layer

The ZigBee application layer consists of the APS sub-layer, the ZDO and the

manufacturer-defined application objects. The responsibilities of the APS sub-

layer include maintaining tables for binding, which is the ability to match twodevices together based on their services and their needs, and forwarding messages

between bound devices. Another responsibility of the APS sub-layer is discovery,

which is the ability to determine which other devices are operating in the personal

operating space of a device. The responsibilities of the ZDO include defining the

role of the device within the network (e.g., ZigBee coordinator or end device),

initiating and/or responding to binding requests and establishing a secure

relationship between network devices. The manufacturer-defined application

objects implement the actual applications according to the ZigBee-defined

application descriptions

ZigBee Device Object Defines the role of the device within the network (e.g., ZigBee coordinator

or end device)

Initiates and/or responds to binding requests Establishes a secure relationship between network devices selecting one of

ZigBees security methods such as public key, symmetric key, etc.

Application Support LayerThis layer provides the following services:

Discovery: The ability to determine which other devices are operating inthe personal operating space of a device.

Binding: The ability to match two or more devices together based on theirservices and their needs and forwarding messages between bound devices


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The Inevitable Question is whether ZigBee and

Bluetooth are competitors or complements?

Bluetooth seems best suited for: Synchronization of cell phone to PDA

Hands-free audio

PDA to printer

While ZigBee is better suited for: Controls

Sensors

Lots of devices

Low duty cycle Small data packets

Long battery life is critical

Air Interface comparison:ZigBeeDSSS

11 chips/ symbol

62.5 K symbols/s

4 Bits/ symbol

Peak Information Rate

~128 Kbit/second

BluetoothFHSS

1600 hops / second

1 M Symbol / second

1 bit/symbol

Peak Information Rate

~108-723 kbit/second

Battery Drain comparison to Bluetooth

Packet length can affect battery drain. Typically the shorter the packet the

quicker the device can go to sleep. Bluetooth is a slotted protocol.

Communication can occur in either: 625 S, 1875 S, or 3125 S slots.The following graph showing effective data rate was based upon the

transmissions speeds stated in Bluetooth v1.1 and IEEE 802.15.4 draft 18,

using the 250 kb/s rate. The general trend is that at larger packet sizes the

effective data rate approaches the raw data rate.

The peaks for the Bluetooth rate are a result of the three slot sizes, when a

packet becomes too big for one slot it must increment to the next slot even


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though it doesnt fill the whole slot allocation.

IEEE 802.15.4 was designed for small packets so it is no surprise it is more

efficient at those small packets resulting in a higher effective rate despite its

lower raw data rate.

From this graph we can see that for packets less than 75 bytes ZigBee has a

higher effective data rate than Bluetooth. Having a lower rate for small

packets means that BT needs longer transmit and receive times and therefore

current drain is higher for small data packets.

Although these numbers do not represent retransmissions or multiple devices

requesting the bandwidth; the author believes that the same traits will be

exhibited in these other cases.

Effective Data Rate

(based upon theoretical values with no retransmissions)Effective Data Rate vs Packet size

0

100

200

300

400

500

600

700

800

1 12 23 34 45 56 67 78 89 100 111 122 133 144 155 166 177 188 199 210 221 232 243 254 265 276 287 298 309 320 331

Packet Size (bytes)

DataRate

(kb/s)


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Timing ConsiderationsZigBee

New slave enumeration = 30ms typically

Sleeping slave changing to active = 15ms typically

Active slave channel access time = 15ms typically

Bluetooth New slave enumeration = >3s, typically 20s

Sleeping slave changing to active = 3s typically

Active slave channel access time = 2ms typically

Conclusion:ZigBee devices can quickly attach, exchange information, detach, and then

go to deep sleep to achieve a very long battery life. Bluetooth devicesrequire about ~100X the energy for this operation.

Power Considerations

ZigBee 2+ years from normal batteries

Designed to optimize slave power requirements

Bluetooth Power model as a mobile phone (regular daily charging)

Designed to maximize ad-hoc functionality

Since IEEE 802.15.4 uses a CSMA-CA protocol the end

nodes only talk when they have data to send with the

following benefits: No waiting for polling (however they must wait for a clear channel

which shouldnt be a problem in low duty cycle networks such as with

sensor and control devices)Current drain is substantially reduced over a polling protocol that must

poll to maintain latencies even though the majority of the time the

device needed be polled

IEEE 802.15.4 protocol was designed to yield 6 months to 2 yrs on

alkaline cell


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ZigBee Battery DrainIn this section well look at different aspects of a networked devices battery

drain.

A typical scenario for sensors and control devices is to remain connected

to the network. We use connected to mean that the device periodicallylistens for incoming packets. In this manner the devices behavior may be

altered or at least checked to verify correctness.

Scenario 1: ZigBee Battery Drain, network connectionLets review a couple of aspects for ZigBee devices:

Goal: Two year battery life

Assumptions:

AAA cell = 1.15 Ahr (Duracell alkaline)

2 yrs = 17,532 hrsPartial result: Average current drain < 65 A (capacity/time)

Tx/Rx current drain ~ 15 mA and sleep current = 1 A

Partial result: Maximum duty cycle < .43% (Avg. current drain-sleep

current)/current drain

Beacon duration of 3 mS (longer beacons containing more information

would drain more current)

Beacon rate of 1/s (beacon rates can be as slow as .03/s)

Partial result: beacon use in this case requires a .3% duty cycle

Final result: 22.8 hours (0.13%) of transmission time would be allowedfor data transmission or reception

Scenario 2: Battery Drain when the unit is not connected to

the networkThis mode can be used to maximize battery life. The device will only

connect to the network when it needs to send data. A disadvantage of this

technique is that the device cannot be sent data, so for the most part it is

seldom part of the network.

Assumptions: Device will connect only when necessary to send data

Acquisition time

Bluetooth requires about 20 30 seconds (~98% confidence) for an

Inquiry (first time) and about 3 seconds for a Page (subsequent times)

IEEE 802.15.4 acquisition time is about 30 mS

Using maximum duty cycle of .43% and 40 byte packet


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- 20 Kinney Consulting LLCki @i

Result:

~ 45,140 data transmissions for Bluetooth

~ 4,269,670 data transmissions for ZigBee

Battery drain conclusion: ZigBee has an inherent advantage for these modes

of operation due to its short attach time and/or its ability to remain in the

sleep mode for long periods.

Comparison Summary ZigBee and Bluetooth are two solutions for two different

application areas. The differences are from their approach to their desired application.

Bluetooth has addressed a voice application by embodying a fast

frequency hopping system with a master slave protocol. ZigBee

has addressed sensors, controls, and other short message

applications by embodying a direct sequence system with a star or

peer to peer protocols.

Minor changes to Bluetooth or ZigBee wont change their inherent

behavior or characteristics. The different behaviors come from

architectural differences.

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